1. Field of the Invention
This invention relates to a method and apparatus for silver recovery during photographic processing.
2. Description of the Prior Art
In the processing of photographic films and papers, fixing or blix solutions are commonly used, the main ingredient of which is a thiosulfate salt.
The fixing solution functions to remove the silver salt remaining in the photographic material after exposure without its reduction to metallic silver by the action of the developer fluid. The silver salt not reduced is dissolved into the fixing fluid in the form of a silver thiosulfate complex anion. On the other hand, the blix fluid is used for rapid processing of certain types of photographic color materials whereby a ferric salt of ethylenediaminetetraacetic acid is included therein in addition to the thiosulfate salt to reoxidize the silver formed by the reduction by the developer, and the oxidized silver salt is removed together with the original silver salt not reduced during the preceding development, both being dissolved and removed from the photographic material in the form of silver thiosulfate complex anion.
Accordingly, the fixing or blix fluid employed for photographic processing contains accumulated silver salt from the photographic material in the form of silver thiosulfate complex anion. Usually the silver content amounts to from 1 g/l to 10 g/l and, in some instances, up to about 20 g/l.
In most cases, the photographic material is washed with water subsequent to fix or bleach-fix. Such washing operation completely removes the residual fix or blix fluid from the photographic material, and thus prevents the silver or dye image resulting in the photographic material from discoloration during prolonged storage. Thus, the fix or blix fluid containing silver complex salts that is carried over by the photographic material into the water wash tank is finally discarded, in a diluted condition, with the wash water waste. Since the water wash is maintained so as not to raise the chemical content thereof by means of continuous supply of a large amount of fresh water, the concentration of silver in the wash water does not usually exceed the level of 1 mg/l to 20 mg/l. However, the total amount of the silver discarded together with the wash water is not negligibly small, in view of the large volume of wash water employed. To recover this amount of silver from the wash water is self-evidently of high economic significance.
Conventionally, however, no practical method of recovering such silver from wash water has been established, thus permitting uneconomical loss of silver.
Recently, the method for recovering the silver from the large amount of waste water containing a very small amount of the silver (for example, a wash water waste) using an anion exchange resin has been proposed. However, it was reported that the capacity of said anion exchange resin was deteriorated with deposition of such an organic compound as gelatin or etc., which was eluted from photographic materials for a short period (Daniel G. Marsh, "Removal of Residual Silver from Processing Waste Water by Ion Exchange", Journal of Applied Photographic Engineering, Vol. 4, No. 1, pp. 17-22 (1978)). Also, form the experimentation performed by the inventors of the present invention, the method described above cannot be practiced economically, since when it recovered by ion exchange resin, the silver thiosulfate complex anion was gradually changed to silver sulfide which did not elute from the ion exchange resin, and reclaiming is implssible.
Further, the method for recovering the silver using a reverse osmosis film was considered, but this method cannot be practiced since the apparatus is expensive.